Prairie Moon Nursery has a massive collection of over 700 North American seeds and plants. It has an excellent plant finder and displays the Native Range of plants in addition to invasiveness.
Prairie Nursery is a wonderful seller of U.S. Native Plants and Seeds, you can search in a variety of categories. In addition their website is packed with helpful information.
Native American Seed specializes in North American plains plants. They have a small but mighty selection of native wildflowers, grasses, unique conservancy species and mixes.
The Joyful Butterfly's focus is on butterfly host plants of the United States. An excellent source of information on planting for butterflies and caterpillars.
Grow Milkweed Plants is an excellent resource of all things Milkweed. You can search for native Milkweed species throughout Canada, the US and Central America by State Province or Territory.
Ernst Seeds has been focusing on ecological restoration for over 50 years. They specialize in bulk seed and have a pretty spectacular seed finder on their website.